This invention relates to a method and apparatus for calibration of fluid flow meters, such as gas meters and the like.
Gas meters presently employed by the gas utility industry are mechanical meters of the double bellows type. For the purpose of calibrating such gas meters, performance data is gathered in such quantity and quality so as to enable corrections to be made to the meter. In practice within the gas distribution industry, data for calibration is obtained by running a proof test. A proof test is comprised of 2 test runs, one at maximum flow rate (open) and one at approximately 30 percent of the maximum flow rate (check). The tests for proof provide calibration data to enable the meter technician to adjust the mechanism of a diaphragm meter that fails the proof tests.
In conducting proof tests for gas meters, apparatus commonly referred to as a bell prover system is used to measure a unit volume of fluid which is passed through the meter under test and the fluid flow volume measured by the gas meter is compared with the known volume of fluid passed through the meter to determine the accuracy of the meter. In such a system, a copper bell of accurate dimension is allowed to descend at a constant rate into a tank of light oil or water. As the bell descends, a suitable test fluid, typically air or natural gas, is passed from the bell through the meter under test. The volume of air or natural gas which is passed through the meter is determined by the amount of linear movement of the bell. The position of the bell accurately defines the volume of test fluid which has been passed through the meter under test.
Typically, in proving gas meters using bell prover systems, a source of air or natural gas is connected to the meter under test and the flow rate of the gas meter is adjusted by selecting a suitable orifice which is connected in series with the source of test fluid and the meter under test. With the flow rate of the gas meter set, the proof run is initiated. With gas meters presently available, initiation of a proof run is effected by interrupting a light using the calibration dial of the meter. At the start of the proof run, the test fluid supply is switched rapidly to the bell. After a known amount of test fluid, typically 1, 2 or 5 cubic feet in present practice, dependent upon meter size, has passed through the meter under test and the bell, the light source is interrupted because the calibration dial has registered one complete revolution. When the light source is again interrupted, the fluid outlet of the bell is closed off, terminating the supply of the test fluid to the meter.
The position of the bell is then accurately recorded electronically, yielding the exact amount of the test fluid that has passed through the meter under test during the time it recorded passage of one cubic foot of fluid as indicated by the calibration dial of the meter. From this measurement, the accuracy or proof of the meter can be calculated. The information obtained can be used to adjust the mechanical mechanism of a meter that fails the proof test.
In the U.S. Pat. No. 4,918,995 which issued on Apr. 24, 1990 to Pearman et al and is entitled ELECTRONIC GAS METER, and which is assigned to the assignee of this application, there is disclosed a gas meter which includes a solid state sensor and solid state signal processing circuits for measuring gas flow volume. This gas meter does not have a calibration dial available for controlling a proof test in a manner similar to mechanical meters of the double bellows type as described above. Also, the meter does not have a mechanical adjustment to improve its accuracy. Thus, proof test techniques and calibration adjustments heretofore used for mechanical gas meters cannot be used on electronic gas meters of this type.
Moreover, the circuitry of the electronic gas meter is powered from a battery. In order to conserve battery energy, the meter employs a sampling technique, and the meter circuitry includes a timing function which defines meter operating cycles. The solid state sensor and associated processing circuits are energized only during a portion of each meter operating cycle for as much time as is necessary to maintain an accurate measurement of volumetric gas flow rate through the meter. The circuitry is energized during active periods or sampling intervals when flow measurements are conducted. The meter circuitry is held inactive for the balance of the meter operating cycle. The meter circuitry does not continuously measure gas flow rate, but rather operates to average flow sample signals produced during successive operating cycles to provide flow rate measurement data from which incremental total volumetric flow is calculated.
In the case of the electronic gas meter, the meter has a definite response curve to the flow rate through it. Therefore, it is necessary to collect enough data so as to derive a transfer function whose input is the solid state sensor's response signal and where output is the flow rate of the gaseous fluid.
Thus, it would be desirable to have a method and apparatus for calibrating a fluid flow meter of the type incorporating solid state sensing and signal processing circuits.